In mathematical writing, I've often seen people use the expression 'Then are equivalent' to introduce a list of conditions that are logically equivalent to each other (and I've used it myself a few times). E.g., here is an excerpt from the bottom of p. 6 (right column) in a 2015 open-access article from Nature Communications (I'm not sure about the nationality of the authors, though):
Proposition II. [...]. Then are equivalent
(1) For a large enough $A$, and corresponding $A'$, there exists a trace-preserving unital map [...] such that [...].
(2) For a large enough $B$, and large enough $B'$, there exists a trace-nonincreasing subunital map [...] such that [...].
And here is another example, from p. 253 in a paper of W.-D. Heinrichs (a German scholar) appeared in Publ. RIMS, Kyoto Univ., Vol. 33 (1997), 241-255:
Theorem 4.8. [...]. Then are equivalent:
[...] satisfies the density condition (DC).
[...] satisfies the strong dual density condition by operator (SDDCO).
[...] is a bornological (DFO)-space.
But I'm a non-native speaker, and a colleague from the US has recently questioned the correctness of the expression, arguing that he finds it "strange, and possibly ungrammatical."
I'd say that the expression is not so uncommon in (mathematical) papers authored or co-authored by native speakers, but I don't have sufficiently robust statistics to advocate for it. So I'd appreciate to hear from the forum on this issue.